1. Field of the Invention
The present invention relates to a display device adapted to display high-quality images, using high-speed, large amount of image data, such as HDTV and, more particularly, to an electrooptical liquid crystal display.
2. Description of the Related Art
The configuration of the prior art system for providing a display of an image is shown in FIG. 20. This system has an image reader 2001 such as a video camera. This image reader scans a desired image, which may be a still image or moving image, and produces output data. A display device 2002 such as an electrooptical liquid crystal display provides a display, using the output data from the image reader 2001, i.e., according to results of the scan, under control of a control unit connected between the display device 2002 and the image reader 2001.
An electrooptical active matrix liquid crystal display which is one example of the aforementioned display device is next described by referring to FIG. 21. This conventional active matrix liquid crystal display comprises a gate-side driver 2116, or a scanning line driver circuit, a source-side driver 2115, or a signal line driver circuit, and a pixel matrix 2105 consisting of a plurality of pixels arranged in rows and column.
The scanning line driver circuit 2116 is composed of a shift register 2102 and a sampling circuit 2103 consisting of complementary TFTs. The shift register 2102 comprises master-slave flip-flops consisting of complementary TFTs.
The scanning line driver circuit 2116 is composed of the shift register 2102 and a buffer circuit consisting of complementary TFTs. The shift register 2102 comprises master-slave flip-flops consisting of complementary TFTs.
The configuration of each pixel is shown in FIG. 22. An N-type TFT 2200 has a gate electrode 2202, a source electrode 2201, and a drain electrode 2203. A liquid crystal element 2204 and an auxiliary capacitor 2206 which are connected to the source electrode 2201 of the N-type TFT 2200 are connected with a counter electrode 2205 and ground 2207, respectively.
The operation of the prior art electrooptical active matrix liquid crystal display constructed as described above is described below. First, the operation of the driver on the gate side, or the scanning line driver circuit 2116, is described. When a start pulse on the gate side and a shift clock pulse on the gate side are entered, a gate signal line 2108 which is connected with a buffer 2107 goes low (L) and then high (H) in synchronism with the shift clock pulse on the gate side.
The operation of the driver on the source side, or the signal line driver circuit 2115, is next described. When a start pulse on the source side and a shift clock pulse on the source side are entered, a sampling signal line 2117 makes a transition from a low (L) level, to a high (H) level, and then to a low (L) level in synchronism with the shift clock pulse on the source side. An image signal entered through an analog RGB signal line 2110 is sampled according to the signal obtained from the sampling signal line 2117, and data about an image is supplied to source signal lines.
The whole active matrix display operates as follows. In order to write data in one horizontal direction, the data about the image is written to pixels on those horizontal lines whose gate signal lines are at a high (H) level in synchronism with the shift clock pulse on the source side. This operation is repeated vertically in synchronism with the vertical shift clock pulses on the gate side. These operations are performed for one frame of image. In this way, one frame of image is displayed. FIG. 23 is a timing diagram illustrating this series of operations.
The manner in which a display is provided by the prior art structure described thus far has some disadvantages, including: (1) The TFTs of the prior art liquid crystal display have small mobilities; and (2) It takes a long time to write data into liquid crystal pixels. For these and other reasons, it has been impossible to set the horizontal sampling clock frequency at a high value. As a consequence, it has been difficult to achieve high-speed operation. That is, it takes long times to change the states of the TFTs and the liquid crystal.
These undesirable phenomena become more conspicuous as the area of the display screen is increased, i.e., the number of pixels is increased, because a larger amount of data is used.
Today, the amount of data about one frame of image is increased manyfold compared with conventional television, in order to achieve higher image quality as encountered in high-definition TV (HDTV) and EDTV. As the display area is increased, the visibility is improved. Also, a plurality of images can be displayed simultaneously on one display device. Hence, there is an increasing demand for larger area displays. To satisfy these requirements, electrooptical liquid crystal displays have been eagerly required to be operated at higher speeds.